Hello 8-bit-gentlemen and chiptune-ladies, i'm looking for the same thing that OP BUT even if this thread is full of very good links, i don't have found my happiness so i think that the best thing for me (and perhaps for you after all) is to code directly my own audio program for the Macintosh Plus 
I think to use assembly with Macintosh Programmer's Workshop... Certainly the hardest way but the better to manipulate in depth the audio chip of the computer.
SO i ask you if someone knows high detailed specs about this chip (more detailed than the official information "Four-voice sound with 8-bit digital/analog conversion, using 22-kHz sampling rate")
This page is already a good starting point : http://www.mac.linux-m68k.org/devel/plushw.php
I will post here my progress at least to keep this thread alive.